Question

Which are the guidelines for choosing between template duck-typing and pure virtual base class inheritance? Examples:

// templates
class duck {
    void sing() { std::cout << "quack\n"; }
};

template<typename bird>
void somefunc(const bird& b) {
    b.sing();
}

// pure virtual base class
class bird {
    virtual void sing() = 0;
};

class duck : public bird {
    void sing() { std::cout << "quack\n"; }
}

void somefunc(const bird& b) {
    b.sing();
}

Answer 1

With template duck-typing, you are doing static polymorphism. Thus, you cannot do things like

std::vector<bird*> birds;
birds.push_back(new duck());

However, since you are relying on compile time typing, you are a little more efficient (no virtual call implies no dynamic dispatch (base on the dynamic type)).